1. Field of the Invention
This invention relates in general to data storage systems, and more particularly, to a method and apparatus for preventing write operations to a data storage medium in response to the data storage system being subjected to a shock event.
2. Description of Related Art
A typical data storage system includes a magnetic medium for storing data in magnetic form and a transducer used to read and/or write magnetic data from/to the storage medium. A disk storage device, for example, includes one or more data storage disks coaxially mounted on a hub of a spindle motor. The spindle motor rotates the disks at speeds typically on the order of several thousand revolutions-per-minute. Digital information, representing various types of data, is typically written to and read from the data storage disks by one or more transducers, or read/write heads, which are mounted to an actuator assembly and passed over the surface of the rapidly rotating disks.
In a typical digital data storage system, digital data is stored in the form of magnetic transitions on a series of concentric, spaced tracks comprising the surface of the magnetizable rigid data storage disks. The tracks are generally divided into a plurality of sectors, with each sector comprising a number of information fields. One type of information field is typically designated for storing data, while other fields contain track and sector position identifications and synchronization information, for example. Data is transferred to, and retrieved from, specified track and sector locations by the transducers which follow a given track and move from track to track, typically under the servo control of a controller.
Writing data to a data storage disk generally involves passing a current through the write element of the transducer assembly to produce magnetic lines of flux which magnetize a specific location of the disk surface. Reading data from a specified disk location is typically accomplished by a read element of the transducer assembly sensing the magnetic field or flux lines emanating from the magnetized locations of the disk. As the read element passes over the rotating disk surface, the interaction between the read element and the magnetized locations on the disk surface results in the production of electrical signals in the read element. The electrical signals correspond to transitions in the magnetic field.
To reduce system errors, it is desirable to locate the read/write elements within the boundaries of each track during the read and write operations of the disk drive. If the read/write elements are moved toward an adjacent track by an external disturbance, the data in the adjacent track can be corrupted if a write operation is in progress. For example, if the read/write transducers move while the system is writing, the new data may write over the old data on the adjacent track, resulting in an unrecoverable loss of the old data.
Present data storage systems typically prevent head movement by employing a closed-loop servo control system. During normal data storage system operation, a servo transducer, generally mounted proximate the read/write transducers, or, alternatively, incorporated as the read element of the transducer, is typically employed to read information for the purpose of following a specified track (track following) and seeking specified track and data sector locations on the disk (track seeking).
Despite the servo system, data storage systems are susceptible to problems arising from external shock and vibrational loads. An excessive shock or vibrational load (shock event) may cause the read/write elements to move off track, for example, to an adjacent track. If this head movement occurs while the drive is writing data, the old data on the adjacent track may be lost. It is therefore desirable to have a data storage system which prevents data from being lost when the system is subjected to a shock event. Typically servo systems are too slow to prevent at least some data from being lost, particularly if a high frequency shock event were to occur.
Typically systems for preventing write operations when the data storage system is subject to a shock event only inhibit write operations in the presence of the shock event. Oscillations in data storage systems caused by transient shock motion resulting from the excitation of the component modes of the data storage system are not accounted for. That is, when the shock event stops, these systems allow write operations to be performed while post-shock motion or oscillations occur. For example, if the initial offtrack magnitude of the read/write elements caused by a shock event is sufficiently large to be of concern, the data storage system will cause write operations to stop by setting a write inhibit flag. The write inhibit flag is then dropped when the read/write elements are positioned ontrack by the servo system. The read/write elements however are typically positioned ontrack prior to the dissipation of the energy of the shock event. In other words, the read/write elements often oscillate about the track several times before the energy of the shock dissipates. The offtrack that occurs during these oscillations is typically much larger than the initial offtrack because of the gains of the modes that are excited. If the read/write elements then move offtrack again because one or more component modes were excited by the shock, the written data may be unreadable. It is also possible that data on an adjacent track can be overwritten and made unreadable.
It can be seen then that there is a need for a method and apparatus for preventing write operations until the energy of a shock event has dissipated.